PROJECT SUMMARY Our long-term goal is to transform knowledge of nosocomial evolution into successful management strategies to confront the growing problem of methicillin-resistant Staphylococcus aureus (MRSA). Here we focus on understanding the different ways by which community- and hospital-associated MRSA (CA- and HA-MRSA) interact with components of the innate immune system to cause different epidemiology and outcomes of infection. Work by us and others suggests that adaptation of MRSA to hospital conditions often involves an interplay of mutations that coordinately confer antibiotic resistance and attenuate virulence. How the resulting changes affect host?pathogen interactions at the cellular and molecular levels is poorly understood. Given that macrophages are central mediators of MRSA uptake and dissemination, we will identify differential mechanisms governing CA- and HA-MRSA intracellular detection by, and survival in, host macrophages. Our preliminary results indicate that the production of cytolytic toxins, which is repressed in HA-MRSA and enhanced in CA-MRSA, enable intracellular MRSA to overcome the expression of macrophage immunity and enhance pathogen survival. At the same time, attenuated cytolytic activity may be advantageous in certain situations, such as in hospital-associated infections, because suppression of inflammatory activity might avoid detection of the pathogen or limit damage to it by the host immune system. The complexity of the selective forces that drive these traits underscores the need for a comprehensive, systems approach to examine the role of host and pathogen capabilities in determining how MRSA subsets differentially modulate immune responses. Given that host?pathogen interactions are pleotropic and interconnected, analysis of individual genes alone cannot explain the cellular responses to infection, much less the bacterial responses. We leverage the power of systems-level analysis of CA-MRSA, transitional CA-MRSA, and HA-MRSA to understand the interactions between MRSA and macrophages during infection. We will interpret profiling results in the context of measures of pathogen versus host success. These include the fate of intracellular bacteria during infection of macrophages in vitro and in murine models specifically designed to reflect conditions in hospitalized patients (disruption of immune functions permit MRSA strains that lack full virulence to cause infection). We will prepare bacterial mutants of relevant pathways to recreate the capabilities of CA- or HA-MRSA strains. We will also perturb specific networks, both in infected human macrophages and in infected murine models, to identify loci where the pathways can be manipulated. By determining the similarities and differences in the host and pathogen transcriptional programs during macrophage infection, comparative analyses between CA- and HA-MRSA will transform our understanding of the pathogenesis of both forms of MRSA. The output will be a mechanistic understanding of host-pathogen interactions that determine the outcome of MRSA infection. Those findings will provide an analytic framework to help control MRSA.